1. Field of the Invention
This invention relates to fluid flow measurement, and, more particularly, to an electroacoustical flow meter with signal quality monitoring capabilities.
2. Description of the Prior Art
There are numerous techniques for measuring fluid flow rates in open and closed channels. Some of these techniques are embodied in systems of the electroacoustical type which include one or more acoustical transducer pairs. Each pair includes an upstream and a downstream transducer spaced a fixed distance apart. The flow rate is determined by computing the difference between the upstream and downstream transit times of acoustical signals transmitted through the fluid between the transducers. Prior systems of this type are shown, for example, in U.S. Pat. Nos. 3,237,453 and 3,564,912.
More particularly, in these systems, an acoustical signal is transmitted through the fluid by a first transducer and is received by a second transducer spaced a fixed distance D away from the first. If the acoustical signal is transmitted from an upstream transducer to a downstream transducer, the acoustical propagation is aided by the flow of the fluid and the transit time is reduced. The reverse is true if the acoustical signal is propagated against the flow direction. The difference in the effective speed of sound along and against the direction of flow is proportional to the velocity of the flowing medium relative to the axis between the transducers. Thus, by accurately measuring the difference between the upstream and downstream transit times, T.sub.u and T.sub.d, respectively, the flow velocity can be determined. The mathematical relationship for determining the flow velocity V relative to the transducer axis is easily derived and may be expressed as follows: EQU V= (C.sup.2 /2D).DELTA. T
where .DELTA. T= T.sub.u - T.sub.d, C is the speed of sound in the fluid and D is the distance between transducers.
Systems of this type must employ a relatively accurate timer for measuring acoustical signal transit times through the fluid, particularly for channels with low flow velocities. Additionally, the timer must be started and stopped at precisely the right time. The same electrical impulse that is used to activate the transmitting transducer may also be used to start the transit time timer in the system. Thus, it is fairly simple to synchronize the start of the timer with the start of the acoustical signal transmission through the fluid. Problems are encountered, however, in stopping the timer exactly at the time that the acoustical signal is received by the receiving transducer.
Typically, a system of this type utilizes a threshold receiver, connected to the receiving transducer, which emits a timing signal when the output of the transducer reaches a readily sensed point in its waveform, usually the first zero-axis crossing. Assuming, for example, that the transducer output undergoes a positive excursion prior to the first axis crossing, the crossing is detected by sensing a negative polarity in the signal. In practice this is done with a threshold detector whose threshold is set at a sufficiently negative level to avoid false triggering because of noise. Because of the steep slope of the waveform in this region, timing errors resulting from noise-like effects on the threshold level are negligible.
The timing signal is used to stop the timer. As long as the acoustical signals received after each pass through the fluid have substantially the same magnitude, the timing signal provides an accurate indication of the end of the transit time period for each pass. However, in many channels, the flow conditions often change from one pass to another causing the magnitude of the acoustical signals received also to change.
Specifically, inhomogeneities in the fluid properties, such as temperature and salinity, or the presence of obstructions, such as air bubbles, silt and debris, can attenuate and distort a propagating acoustical signal. In open channels, random reflections from the upper and lower surfaces of the channel often combine with a direct path signal to produce a received signal, the magnitude of which is dependent on the phase relationship between the reflected and direct path signals. If the attenuation is severe enough, so that no portion of a propagated signal has an excursion greater than the threshold of the zero-crossing detector, these signals go completely undetected. Also, the signal attenuation may be such that the first zero crossing does not have an excursion greater than the threshold of the zero-crossing detector, but the second zero crossing does, causing the detector to respond to the wrong half cycle of the signal. These factors give rise to instabilities in the receiver timing signals, and thereby result in inaccurate and unreliable flow rate readings.
It is therefore, an object of the present invention to provide a flow metering system of improved accuracy.
Another object of the invention is to provide an electroacoustical flow meter with signal quality monitoring capabilities.
Still another object of the invention is to provide an electroacoustical flow meter of the type described which minimizes flow reading errors due to fluid disturbances.
Yet still another object of the invention is to provide an electroacoustical flow meter of the type described which minimizes flow reading errors due to reflected acoustical signals.